1. Field of the Invention
The present invention relates to a printed board drilling method and a printed board machining apparatus for machining holes in a printed board by using drills.
2. Description of Related Art
The size of holes drilled in a printed board are required to be highly accurate in the depth direction. Japanese Patent No. 2559788 discloses a machining apparatus that calculates a distance between a head of a pressure foot and the tip of a tool in advance of machining and controls a feed distance of a spindle head based on that distance. This technology will be explained below.
FIG. 5 is a front section view of a main part of the conventional printed board machining apparatus.
As shown in FIG. 5, a printed board 1 is fixed on a table 2. A ring 4 is attached around a drill 3. The drill 3 is rotatably held by a spindle 6 through an intermediary of a collet 5. An edge of the collet 5 forms one and same plane with a head of the spindle 6. A spindle head 7 supports the spindle 6 as well as a pair of air cylinders 8. The spindle head 7 is movable in the vertical direction in FIG. 5 by means of a servo motor 9. The height of the head of the spindle 6 from the surface of the table 2 is known in advance.
A pressure foot 10 is supported by rods of the air cylinders 8 and is urged downward in FIG. 5 by the air cylinders 8. First detecting means 11 disposed between the spindle head 7 and the pressure foot 10 detects a relative movement (distance) between the spindle head 7 and the pressure foot 10.
Second detecting means 12 fixed on the table 2 detects the position of the tip (edge) of the drill 3 (from the surface of the table 2, subtracting a length l1 from the surface of the table 2 when the surface of the second detecting means 12 protrudes by the length l1). The first detecting means 11 and the second detecting means 12 are connected with a calculating circuit 13. A control circuit 14 is connected with the first detecting means 11 and the calculating circuit 13 and controls the servo motor 9.
Next, operations of the conventional printed board machining apparatus will be explained.
In replacing the drills, the control unit 14 positions a new drill 3 above the second detecting means 12 and lowers the spindle head 7 by a distance defined in advance.
The spindle head 7 continuously descends even after when the head of the pressure foot 10 abuts against the second detecting means 12 and stops to move. The first detecting means 11 detects the relative movement between the spindle head 7 and the pressure foot 10. When the spindle head 7 stops after descending by a predetermined distance, the second detecting means 12 detects the position where the tip of the drill 3 has reached, i.e., the height of the tip of the drill 3 from the surface of the table 2.
The control unit 14 calculates a distance 12 from the tip of the drill 3 to the head of the pressure foot 10 from a difference between the relative movement of the spindle head 7 and the pressure foot 10 and the position where the drill 3 has reached. Then, the control unit 14 adds a machining depth 13 specified in advance with the distance l2 and sets the relative movement of the spindle head 7 and the pressure foot 10 necessary for machining as a feed distance.
During machining, the control unit 14 compares the distance of the relative movement of the spindle head 7 and the pressure foot 10 when the pressure foot 10 abuts against the printed board 1 with the feed distance and when they become equal, stops or reverses the rotation of the servo motor 9.
Because the control unit 14 thus detects the distance from the tip of the drill 3 held by the spindle head 7 to the head of the pressure foot 10 and controls the movement of the drill 3 from the start of the relative movement of the spindle head 7 and the pressure foot 10 to the end of machining based on the distance and the preset machining depth, it has been possible to drill accurately up to the machining depth l3 defined in advance.
FIG. 6 is a table showing stored contents of record of conventional tool data. Inputted in the tool data table Tb2 stored within the control unit 14 are the Storage Areas (normally tools are stored one by one) of tools, i.e., drills in this case, Tool No., Diameter of Tool, Number of Revolution of Spindle, Cutting Speed and Life of Tool (allowable number of holes to be drilled) for example. As shown in the table, Tool No. identifies the diameter of the tool and each of the tools is specified by Storage Area.
Several thousands to several tens of thousands holes are drilled often in one printed board. Accordingly, a tool cassette for holding a plurality (several hundreds) of tools is disposed at one end of the table for mounting a printed board and holes of 0.1 to 6.5 mm for example are drilled by one printed board machining apparatus while replacing the tools in accordance to a tool holding command issued based on a machining program or the life of tools.
Presently, two kinds of drills having a shank of 3.175 mm and 2.00 mm in diameter are commercially sold for the printed board machining apparatus. A total length of the drill having the shank of 3.175 mm in diameter is 38.1 mm and that of the drill having the shank of 2.00 mm in diameter is 31.75 mm, regardless of their nominal diameter (diameter of cutting edge).
FIGS. 7A through 7C explain conventional methods for holding the drill.
A first conventional method uses the ring 4 made of synthetic resin. The ring 4 is attached to the shank of the drill 3 so that a distance from its end face, i.e., the head of the spindle, to the tip of the drill 3 becomes equal to a drill extension (hereinafter referred to as ‘extension’) A defined in advance as shown in FIG. 7A. The drill 3 attached with the ring 4 is held by a supply discharge post 21 shown by two dotted lines so as to allow the collet 5 to hold its shank.
A second conventional method is to position the drill 3 by abutting the rear end thereof to an end face of a stopper 20 within the collet 5 while holding its tapered face 3t in a supply discharge post 22 as shown in FIG. 7B. The stopper 20 is positioned so that the extension becomes equal to A, i.e., the extension of the drill 3 held by the collet 5 is A. It is noted that when the nominal diameter of the drill is larger than that of the shank, the drill is held in the supply discharge post 21 while attaching the ring 4 to the shank in the same manner with the first method as shown in FIG. 7C. However, it is the same with the method in FIG. 7B in that the drill is positioned by abutting its rear end to the stopper. In this case, a gap g (g>0) is formed between the ring 4 and the head of the spindle 6.
That is, the drill extension is set constant in every case regardless of the nominal diameter of the drills. It makes it easy to control the height of the tip of the drill to the surface of the workpiece when moving the drill 3 in the horizontal direction from the spot just machined to a next machining spot.
By the way, needs for drilling holes of very small diameter is increasing lately. In order to drill the holes of very small diameter, a spindle must be rotated at high-speed and a high-speed spindle (e.g., more than 300,000 revolutions/min.) has been put into practical use.
However, the higher the speed of the spindle, the more a runout z of the tip of the drill increases often as shown in FIG. 8. When the runout z of the tip of the drill becomes significant, not only the machining quality such as positional accuracy of holes and true roundness of drilled holes degrade but also the machining efficiency drops because drills are apt to be broken.